Underreamed (Belled) Piers and Skin Friction

[conradlovejoy]

When I have designed straight shaft piers in the past, I've added what is basically uplift using the compression SF to the allowable load obtained from A_g*σ_allow since that compression skin friction is adding to the pier's resistance to applied loads. (i.e. P_allow = A_g*σ_allow*(Π*D*(penetration into bearing stratum - top of skin friction layer)*SF_compression)

In the case of an underreamed pier, does all of allowable load resistance now come from the end bearing (A_bell*σ_allow), or is that wall skin friction I typically add still an additive force? And of course, if you think I am way off in any aspect, please elaborate.

 

[jayrod12]

You typically get to use 1 or the other. Not both. That's due to the different levels of movement required to mobilize each different type of resistance. I.e. in order to mobilize the end bearing capacity, you will have overcome the skin friction.

 

[JAE]

Every belled pier I have ever designed was based on end bearing only. This was possibly a standard approach of the area in which I worked (south Texas) but was always recommended by the local geotechnical engineers in their reports. We typically had good blue shale about 30 to 40 feet down and expansive clays above that so perhaps the clays were not really conducive to skin friction.

There is also the concern that the excavation of the bell might disturb the soils directly above the bell so some length of shaft directly above the bell (if you were to add in skin friction) would be ignored.

Some of our geotech posters here might chime in but that is what I've seen.

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[GeoPaveTraffic]

For belled piers, I would not recommend that you count on any skin friction. As others have said, use either end bearing or skin friction; not both.

Mike Lambert

 

[conradlovejoy]

Thanks for all of your responses!

 

[conradlovejoy]

Follow-up question:
I agree that skin friction in compression being neglected in terms of resisting applied axial loads is a good practice.

What about those that use the skin friction in tension to resist the uplift forces due to soil swelling? Does anyone agree that the amount of pier below the required embedment into the bearing strata/stable zone can be used in conjunction with the skin friction in tension to resist the applied uplift force? Furthermore, can the amount of bell outside of the shaft diameter (i.e. Π/4[d_b²-d²) in conjunction with the skin friction in tension be added (or is there some geometry regarding the angle of the bell ceiling involved)?

 

[jayrod12]

I would look at using the amount of soil to be lifted by the bell as the resisting mechanism in that case. ANSI/ASAE EP486.1 has a pretty good uplift calculation for situations like this. specifically clause 8.3.2.1

 

[JLNJ]

I have had geotechnical engineers allow for some friction component in addition to the end bearing component (depending on the soil type).

It's worth asking the question.